The system and method of this invention lie in the field of alternating current generation with controlled voltage and frequency to enable connection to a power supply system carrying current at predetermined voltage and frequency. They are directed particularly to the utilization of a prime mover which drives a power input shaft at widely varying speeds with a range extending well above that necessary to produce current at selected voltage and target frequency, which shaft drives one or more generators without modifying control of the shaft speed.
Various schemes have been proposed and utilized for driving and controlling the speed of motors and generators and have been generally satisfactory for the purpose. In the field of electrical current generation the output frequency of the generator has been maintained at a desired value, and hence the angular velocity of the rotor, by comparing the generated frequency with a standard reference frequency and modifying the rotational speed of a driving turbine to cause them to match. Other systems have maintained the proper angular velocity of the rotor by interposing a mechanical variable speed transmission in the drive line between a prime mover and the rotor shaft. It has also been proposed to control the speed of an induction motor by rotating its frame, or stator, at different speeds, an auxiliary motor being used to produce the rotation of the frame. Among examples of these various types of devices might be mentioned the U.S. Pat. Nos. 1,971,818 to Hoxie, 2,039,322 to Lell, and 2,061,983 to Rossman.
While systems such as those mentioned are well suited to installations in which stored energy is used for the power supply and hence the power may be reduced without waste of energy, they are not practical for use when the source of energy cannot be stored and varies over a wide range, such as relative wind driving windmills or air turbines. If the full energy is not used it is lost and cannot be recovered at a later time.